1. Field of the Invention
The present invention relates to a message display system for a programmable controller (PC) for displaying messages on a display screen while the PC is in operation, and more specifically, to a message display system for a PC having a function to display a message for each language.
2. Description of the Related Art
Conventionally, a message display system for a PC has a function for displaying a message for each language. This system is arranged such that an area for storing each language is provided in a ROM contained in the PC and when the PC is put into operation, a language designated on a sequence program is fetched from the ROM to display a message. As a result, even if a PC is shipped to a region using a different language, it is not necessary to design a different sequence program corresponding to the language used in the region.
FIG. 9 (Prior Art) is a diagram showing an example of a ladder program according to a conventional PC message display system. A net L1 includes a message display command (DISP) 91 as a function command. Nets L2-L51 are a net group LN1 for a Japanese message and nets L52-L101 are a net group LN2 for an English message. Further, net groups for a German message and French message are also provided in addition to the above net groups. It is assumed, however, that only the net group LN1 for the Japanese message and the net group LN2 for the English message are programmed to simplify the description.
In the ladder program shown in FIG. 9, when a contact ACT is turned ON, a message display command (DISP) 91 is executed. As a result, a message corresponding to the address of the coil turned ON of coils 92-9101 are displayed. The messages included in the coils 92-951 for Japanese messages have meanings sequentially corresponding to the meanings of the messages included in the coils 952-9101 for English messages. For example, if a message displayed when the coil 92 is turned ON is "MACHIGAI", in Japanese which means error, a message displayed when the coil 952 is turned ON is "ERROR" in English.
As shown by the relationship between the coils 92 and 952, coils corresponding to a message of the same meaning in Japanese and English share a common contact. More specifically, the coils 92 and 952 share a contact X0, the coils 95 and 953 share a contact X1, and further the coils 951 and 9101 share a contact X49.
In addition, contacts OPT1 and OPT2 are connected to all the nets L2-L101. These contacts OPT1 and OPT2 are set to a contact a by a sequence program in the net group LN1 for the Japanese message, whereas the contacts OPT1 are set to the contact a and the contacts OPT2 are set to a contact b in the net group LN2 for the English message. A state of signals input to these contacts OPT1 and OPT2 is preset by parameters by an operator. Note specifically, when a message is desired to be displayed in Japanese, a state of signals input to the contacts OPT1 and OPT2 is set to an ON state, whereas when the message is desired to be displayed in EngLish, a state of signals input to the contact OPT1 is set to an ON state and a state of signals input to the contact OPT2 is set to an OFF state.
Thus, when any one of the contacts X0-X49 is turned ON, the coil on the net are turned ON. When a parameter is set to display the Japanese message and the contact X0, for example, is turned ON, the coil 92 on the net L2 is turned ON. As a result, a message corresponding to the address R0.0 of the coil 92, i,e., "MACHIGAI" is displayed on a screen, whereas when the parameter is set to display the EngLish message and the contact X0 is turned ON, the coil 952 on the net L52 is turned ON. As a result, a message corresponding to the address R5.0 of the coil 952, i.e., "ERROR" is displayed on the screen.
Nevertheless, since the aforesaid conventional message display system includes the coils corresponding to unnecessary other languages in the ladder program, a program processing executed for these coils is very wasteful.